Surfaces of microfeature workpieces may be subjected to a variety of chemical processes during manufacture or in analytical processes, e.g., in quality control testing. As feature sizes on microfeature workpieces get smaller and performance depends more heavily on control of material compositions, microfeature workpieces become more sensitive to variations or contamination during such chemical processes.
For example, etching may be used in manufacturing operations to selectively remove specific portions of the surface of the workpiece, e.g., to form vias or other functional features, or to pattern a blanket conductive layer. There are generally two types of etching processes—“dry” etching and “wet” etching. Most dry etching operations are carried out using a high-energy plasma that may selectively remove the portions of the microfeature workpiece surface. Wet etching processes are generally conducted in a tank that contains a volume of a chemical etchant liquid.
FIG. 1 schematically illustrates a conventional dry etching system 10. The dry etching system 10 includes a vessel 12 that is designed to receive a microfeature workpiece W. A plasma is delivered to the interior of the vessel 12 by a plasma source 20 and is directed toward the microfeature workpiece W. In the embodiment schematically illustrated in FIG. 1, the plasma source 20 includes a gas supply 22 that communicates with a distributor 24 adjacent the top of the vessel 12. A microwave generator 26 or other energy source is placed in line between the gas supply 22 and the distributor 24 to generate a plasma that is directed toward the microfeature workpiece W by the distributor 24. Waste gas is withdrawn from the vessel 12 by an exhaust 16, which may include a vacuum pump.
In many circumstances, the plasma source 20 will provide sufficient energy to etch the surface of the microfeature workpiece W. In some circumstances, though, it may be advantageous to heat the microfeature workpiece W before, during, or after generating the plasma in the vessel 12. For this reason, dry etch systems may include one or more conduction heaters 14. Such conduction heaters 14 typically are placed outside the vessel 12 to separate them from the plasma and are most commonly positioned below the microfeature workpiece W. This requires that the heat generated by the conduction heater 14 be transferred to the microfeature workpiece W through the wall of the vessel 12. To keep process times acceptably short, the conduction heater 14 must heat this vessel wall significantly above the temperature of the microfeature workpiece W. The high energy plasma and the high temperatures employed by the conduction heaters 14 can significantly limit the materials suitable for forming the vessel 12.
FIG. 2 schematically illustrates a conventional wet etching system 50. In this wet etching system 50, a microfeature workpiece W is positioned in the interior of a tank 52 and immersed in an etching liquid 54. Carrying out wet etching at room temperatures may require the use of an overly aggressive etching liquid that makes it difficult to control the etching of the microfeature workpiece. For this reason, many wet etching systems heat the microfeature workpiece and/or the etching liquid during an etching process. The wet etching system 50 of FIG. 2 is schematically illustrated as having two types of heat sources. The first is an external conduction heater 60 that can conduct heat through the wall of the vessel 52 to heat the microfeature workpiece W, similar to the conduction heater 14 illustrated in FIG. 1. Either in addition to or instead of such an external conduction heater, the wet etching system 50 may include one or more internal heating elements 62. Such internal heating elements 62 can reduce thermal lag in the system and afford greater control over the etching process. Each of these heat sources has its disadvantages, though. As noted above, an external conduction heater 60 requires the wall of the tank 52 to be able to withstand elevated temperatures that exceed the maximum temperature of the etching liquid 54. This can significantly restrict the choice of materials for forming the tank 52. The internal heating elements 62 are in direct contact with the etching liquid. Any contamination from the heating elements 62 in the etching liquid 54 may contaminate the microfeature workpiece W. To limit this contamination, internal heating elements 62 are typically coated with a material that is substantially non-reactive with the etching liquid. Any coating defects that are initially present or that develop over time can still lead to contamination of the etching liquid 54, requiring frequent inspection and maintenance of the internal heating elements 62. In addition, the internal heating elements 62 typically require an electrical or other connection through a wall of the tank 52. Seals may be formed around these connections to limit any leakage or contamination of the etching liquid 54, but such seals are subject to degradation and present another maintenance requirement and potential point of failure.
As noted above, chemical processes may also be used in analyzing aspects of microfeature workpieces. For example, it may be desirable to etch or partially “digest” a microfeature workpiece W as a step in chemically analyzing a microfeature workpiece W. A so-called hot wafer digester, for example, may employ a system similar to the wet etching system 50 to dissolve a layer or film on a surface of the microfeature workpiece W. The resultant contaminated etchant liquid 54 may be analyzed using known analytical chemistry techniques to determine aspects of the composition of the film or other material removed from the microfeature workpiece W. If internal heating elements 62 are employed, though, the potential contamination from these heating elements 62 can reduce the reliability of such chemical analysis and may effectively preclude the detection of ultratrace concentrations of specific components in the material removed from the microfeature workpiece W. If an external conduction heater 60 is used instead of internal heating elements 62, the tank 52 typically must be formed of a high temperature material. One of the most common high temperature materials for hot wafer digesters is quartz. Commercial availability of quartz tanks sufficiently large to handle 300 mm-diameter wafers is quite limited, at least in part due to the difficulty and expense of manufacturing large vessels of high-purity quartz. In addition, hydrofluoric acid, a common semiconductor etchant, generally can not be used in a quartz vessel because it will attack the quartz.